Whitehead and Worthington (1982) have measured the fluxes of heat and salt due to the northward flow of Antarctic Bottom Water through a passageway 300 km wide between the Ceara Rise and the Mid-Atlantic Ridge at about 4 ¿N. Downstream of this ''sill'' the temperature and salinity of the underflowing water increase, and Whitehead and Worthington have described this change as being due to ''downward'' fluxes of heat and salt across isothermal surfaces. We consider the relative roles of three separate mixing processes to these ''downward'' fluxes of heat and salt across isotherms and we use this information to decide between Whitehead and Worthington's two separate estimates of the volume transport of Antarctic Bottom Water: one based on current meter data, and the other based on geostrophic calculations. The slope of the &thgr;-S locus of bottom water as it moves northward past the equator provides a valuable extra constraint on the relative importance of the three mixing processes. We concluded that the dominant mixing process is diapycnal (i.e., cross-isopycnal) turbulent eddy diffusion and that the geostrophic data set of Whitehead and Worthington is consistent with the mixing ideas presented here, whereas their current meter data set is not.